The present invention relates to a thin, biaxially oriented polyester film which is asymmetrically oriented by stretching 1.0-2.0 times in the machine direction in the plane of the film and about 3.0 to 5.0 times in the transverse direction in the plane of the film, said film having a birefringence of at least 0.085, an optical retardation of at least 9000 nanometers, a shrinkage of less than 2% in any direction at 150.degree. C., which is substantially free of color fringes upon viewing in partially polarized light.
This invention also relates to two processes for the production of said biaxially oriented film.
The film of the invention may be bonded to materials, such as polyvinyl butyral, to form a two-ply or three-ply laminated structure, which further may be bonded to a glass layer or layers of a glazing structure to provide the desired optical and mechanical characteristics. The film of the invention can also be primed with adhesion promoters or other surface modifiers at any suitable stage during manufacture of the film, i.e. before or during the stretching operation, or it may also be applied to the finished film. Polyester films such as disclosed in U.S. Pat. No. 4,898,786 are contemplated.
The use of polymeric films in glazing applications is well-known. Oriented polyester film, particularly biaxially oriented film composed of polyethylene terephthalate (PET) has been widely used as a packaging material or as a base for microfilm, reprographic films, proofing films and the like. It has good optical clarity and toughness which renders it extremely suitable for such applications.
Polyester films can also be used as transparent and translucent layers applied to solid substrates and particularly to transparent substrates. Another more recent application for polyester film involves its use as an antilacerative layer in the construction of automotive glazing products such as windshields, rear windows, sun roof panels and the like, and in architectural safety glass. One of the simpler automotive glazing products may comprise a flexible interlayer of polyvinyl butyral sandwiched between glass on one side and a high modulus film, such as polyester film, on the opposite side.
The introduction of laminated safety glass for automotive glazing structures substantially reduced the hazard from flying glass shards. Such structures consist of two layers of glass bonded to a central layer of a tough, stretchable material such as plasticized polyvinyl butyral. These structures are designed to have the strength to allow bending but not penetration by the head of an occupant upon impact in a collision. Lacerations may occur if the head slides over the fragmented surface of the inner glass layer after impact; therefore, a layer of this tough, stretchable material used as the inner layer can prevent this source of injury.
One of the major problems in laminated safety glass is the "rainbow effect". If the material of any component of the glazing structure in the viewing system is slightly anisotropic to partially polarized light, color distortions are possible. These distortions are known as color fringes or the "rainbow effect". The requirement of freedom from color fringes when viewed in partially polarized daylight is a critical one. Additionally, an "elephant skin" effect is possible when there is excessive shrinkage of the film during the lamination process.
The preferred film of this invention has a higher birefringence then similar films known; it is this property that helps solve the problems of the "rainbow effect".
Although the use of polymeric films in glazing applications is known, the problem of manufacturing films possessing the properties of optical clarity, thermal stability, formability and the like, has not been successfully addressed.
Canadian Patent No. 596,193 to Gore et al., discloses one attempt at addressing these problems. This patent is directed to a safety glass laminate having a number of layers, one of which is a sheet of biaxially oriented polyethylene terephthalate. While the film disclosed therein possesses a number of the desired properties sought in windshield use applications, it lacks other properties, such as frangibility and freedom from color fringes or "rainbow" effects as well as "elephant skin" effects.
U.S. Pat. No. 4,072,779 to Knox et al. discloses a polyethylene terephthalate film which claims to have some of the desired properties however the film produced under the conditions of this patent but is extremely difficult to make as a heat stabilized film. The film produced is difficult to manufacture, process and handle because of the poor machine direction mechanical properties resulting from the conditions specified therein.
Means and materials for achieving this required optical property, in combination with other desired mechanical, thermal and optical properties, are the goals the present invention seeks and achieves.